Cleaning exhaust gases from combustion installations

ABSTRACT

In a process for cleaning the exhaust gases from combustion installations, a dilute urea solution prepared in a reagent tank (20) is introduced into the hot exhaust gas flow (30) of differing concentrations and is finely sprayed in the direction of the exhaust gas flow. After decomposition of the urea in a pyrolysation channel (14), the exhaust gas flow (30) is homogeneously mixed in-line by a mixer located in a mixing channel (16). In a subsequent in-line reaction channel (18), the reducible exhaust gas constituents are converted into non-toxic gases in at least one selective reduction catalyst which does not contain zeolite (36), then, dependent on the installation, the oxidizable exhaust gas components are converted into non-toxic gases without reagent in at least one oxidation catalyst (38) to produce a virtually complete reaction. A dual substance nozzle appliance (26) opening into the pyrolysation channel (14) comprises a reversing valve (88) for the working and blowing out position, a casing tube (24) for the compressed air, located in the area of the exhaust gas flow ( 30), a urea conductor carried at a distance in the casing tube (24) and a nozzle for fine spraying of the dilute urea solution.

The invention relates to a process of cleaning exhaust gases fromcombustion installations, in particular from diesel, injection andgas/diesel installations, large gas petrol engines, gas turbines andboiler installations which are fired by liquid, gas or solid fuels,through the introduction into the hot flow of exhaust gas of a diluteurea solution of differing concentrations prepared in a reagent tank andan at least single stage catalytic reaction of the toxic, gaseousexhaust gas constituents with a reduction stage or a reduction and anoxidation stage. The invention also relates to an exhaust gas cleaninginstallation with a reagent tank, a metering and feed device for thedilute urea solution and an electrical control and regulation unit toimplement the process.

Catalysts in the exhaust gases of combustion installations are ofincreasing importance, and in many countries they constitute a necessaryprerequisite in order to comply with legal standards.

Combustion installations with maximum levels of efficiency whichsimultaneously eliminate toxic gases by means of catalysts are the mostenvironmentally friendly and energy-saving. The useful power (level ofefficiency) extracted per kilogram of oil or per cubic meter of gas canbe decisive in determining whether a system is sensible andenvironmentally friendly from an energy point of view. Maximumconversion rates of toxic chemicals can only be achieved by the use ofcatalyst technology.

Catalysts are divided into two main groups:

Three-way catalysts, which are used in operation without excess air, andare not of interest here.

SCR (SCR=Selective Catalytic Reduction) catalysts, which are used inexhaust gas cleaning installations of combustion installations. Highnitric oxide values at or below the limits required by the legislatorscan be achieved with SCR catalysts. For NOx reduction, a reagent is usedin addition to a selective reduction catalyst. Ammonia is highlysuitable as a reagent, although it is a problematic medium as regardstransport, storage and handling. Urea is therefore also used as asubstitute for ammonia. This is supplied as a white, dry granulate or asa ready-to-use solution, it is non-toxic, odour-free and poses noproblems as regards storage and transport.

In DE,A1 3830045, a process is proposed for the reduction of nitricoxides contained in exhaust gases, in particular those of dieselmachines, under oxidation conditions by means of a catalyst containingzeolite, whereby a substance containing urea is added to the exhaust gasas a reduction agent. Metering of the reduction agent may bestoichiometric, under or over-stoichiometric. DE,A1 3830045 aims tocreate a way of reducing nitric oxides in exhaust gases, avoidingproblematic reduction agents, using a catalyst which contains zeoliteeven where the exhaust gas has a low hydrocarbon content. The catalystwhich contains zeolite is therefore of fundamental significance forDE,A1 3830045.

The inventors have sought to solve the problem of creating a process forcleaning exhaust gases and an exhaust gas cleaning installation forimplementing the process, of the type mentioned at the outset, whichguarantees as complete a disintegration as possible of the urea intoammonia and carbon dioxide or its conversion into ammonia, cyanuric acidand carbon dioxide, before the water droplets containing urea come intocontact with a hot metal surface in the exhaust gas flow and formunacceptable deposits. No waste products requiring disposal areproduced, but virtually exclusively non-toxic gases with the aid of acatalyst which does not contain zeolite, without impairing the highrates of effluent conversion and economically viable operation.

With respect to the process, the problem is solved according to theinvention in that in a program controlled exhaust gas cleaninginstallation, the dilute urea solution added is injected into apyrolysation channel in a fine spray using a flow of compressed airfinely sprayed towards the exhaust gas flow, homogeneously mixed in-lineusing a mixer located in a mixing channel, and at least the reducibleexhaust gas components are converted into non-toxic gases in a reactionchannel, using the decomposition product (NH₃) of the dilute ureasolution in at least one selective reduction catalyst which does notcontain zeolite, or additionally the oxidisable exhaust gas componentsare also converted into non-toxic gases without reagent in at least oneoxidation catalyst, to produce a virtually complete reaction. Specialand further types of design of the process according to the inventionare the subject of dependent patent claims.

By a virtually complete reaction it is understood that toxic gases areconverted at a high level of efficiency, preferably greater than 80%, inparticular greater than 90%, dependent on substance and temperature. Thedilute urea solution is preferably directed into the exhaust gas flowunder a delivery pressure, i.e. under a slight overpressure. Theaerosol-type atomization takes place through air with an overpressure ofappropriately 0.2 to 8 bar, preferably 0.5 to 6 bar, and in the case ofsmall installations, 0.5 to 1.5 bar. This causes the dilute ureasolution flowing out to be entrained and atomized through the injectoreffect. The exhaust gas channel should preferably be dimensioned suchthat the exhaust gas preferably flows at a speed of 15 to 60 m/sec.

However the compressed air does not only serve as a means of conveyance,but also as a coolant. The compressed air is carried along the ureaconductor in such a way that the temperature of the urea solution in theurea conductor and up to the outlet opening of the nozzle is preferablymaintained at a maximum of 100° C. At higher temperatures the ureabegins to decompose before atomization of the solution, which is notdesirable.

According to a further design form, the dilute urea solution is not onlysprayed by the compressed air after leaving the air nozzle which itselfproduces a twist effect, but is simultaneously rotated. Distribution ofthe sprayed reagent in the exhaust gas flow is thereby further improved.

In order to achieve an even and complete reaction over the entire crosssection of the exhaust gas without any local reagent surplus, thereagent is preferably fed in fully automatically, preciselystoichiometrically proportioned to the NOx mass flow.

After homogeneous mixing in the mixing channel, the exhaust gas flow inthe reaction channel, which is homogeneously mixed with the atomizeddilute urea solution or its decomposition products, which principallycomprise ammonia (NH₃), is directed along the many lengthwise channelsof the catalysts in a steady flow and in the free reaction channel in aturbulent manner. This may take place both in the reduction stage and inan optional oxidation stage.

With respect to the NOx conversion, the highest possible selectiveaction SCR catalysts are used. Oxidation catalysts may act selectivelywith respect to the CO, HC and/or SO₂ components. Such catalysts aredeliberately selected and used.

When switching on the exhaust gas cleaning installation, it isappropriate to first blow in compressed air so that the urea conductorand nozzle are well cooled. Then the dilute urea solution is switched onat the predetermined dosage rate by operating a three-way valve. At theend of the reaction, the flow of dilute urea solution is firstinterrupted, again by operating the three-way valve. Finally, compressedair is fed to the urea conductor by a further operation and the nozzleis blown clean, in order to avoid the formation of any residues.

The volume of the compressed air flow is measured and monitored. As soonas it falls below a prescribed minimum, the supply of the dilute ureasolution is stopped in order to prevent it from entering the exhaust gasflow without being atomized, or else not fully atomized.

Should the nozzle become blocked during the reaction, the pressureincreases immediately in the urea conductor and/or in the casing tubefor the compressed air. If the level should fall below a prescribedthroughput of air or urea solution and/or in the event that a prescribedoverpressure is exceeded, then sensors trigger the immediatedisconnection of the dilute urea solution supply.

When the reagent supply is switched off, compressed air is immediatelyfed into the urea conductor and it is blown out. In the event of astoppage, the pressure gradually increases, for example within 30seconds, up to the compressor pressure. In normal circumstances thestoppage is blown out during this pressure rise and the nozzle iscleaned. The reagent supply remains switched off until normal conditionsare restored.

If the blockage cannot be blown out with compressed air, then simplemechanical means are used for cleaning, as will be shown later.

With respect to the exhaust gas cleaning installation for implementationof the process, the problem is solved using the invention in thefollowing manner:

the feed device is designed as a dual substance nozzle appliance andopens out into the pyrolysation channel, and comprises a reversing valvefor the working and blowing out position, a casing tube for thecompressed air arranged in the exhaust gas flow area, a urea conductorin the casing tube at a distance and a nozzle for fine spray of thedilute urea solution,

at least two nozzle mixers are fitted at a distance apart in the mixingchannel, and

at least one honeycomb reduction catalyst designed with lengthwisechannels or at least one reduction catalyst and at least one oxidationcatalyst are fitted in the reaction channel facing towards the exhaustgas flow. Special and further design forms of the exhaust gas cleaninginstallation are the subject of dependent patent claims.

A reduction catalyst is always fitted during the process, and anoxidation catalyst is only fitted when required.

The advantages of cleaning exhaust gases according to the invention maybe summarised as follows:

No waste products requiring disposal are formed, only non-toxic gases.

Thanks to the dual substance nozzle appliance, the dilute urea solutionmay be injected in with maximum operating safety, in such a way that theurea decomposes completely. The compressed air acts simultaneously as atransport, cooling and blow-out medium.

The catalysts are recyclable. They may be fitted and removed manuallyfrom the catalyst cases without lifting appliances.

The installation may be ideally adapted to suit the spatial conditions,since it is designed extended or compact, horizontal, vertical ordiagonal.

The exhaust gases of all combustion processes in engines, boilers andturbines which work with excess oxygen may be cleaned.

The injection appliance operates in a low to medium pressure range, thedilute urea solution is supplied with low conveyance pressure, thecompressed air for cooling and spraying is supplied with an excesspressure of only 0.2 to 8 bars.

The injection appliance automatically switches over to self-cleaningwith compressed air through the operation of a three-way valve, in theevent of a blockage or when work is complete.

The nozzle mixer fitted in the mixing channel is preferably at adistance in the range between (1.5-2.5)×d_(h). The hydraulic diameterd_(h) corresponds to four times the inner surface area of the pipeconcerned divided by its circumference. The use of an oxidation catalystdepends on the installation and the exhaust gas concerned, and may beomitted if circumstances and requirements permit.

The invention is explained in greater detail using the design examplesshown in the drawing, which are also the subject matter of dependentpatent claims. The diagrams show the following:

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a cleaning installation with compactly arranged channels,

FIG. 2 a cleaning installation with channels arranged in an extendedmanner,

FIG. 3 a perspective view of a pyrolysation channel,

FIG. 4 a perspective view of a mixing channel,

FIG. 5 a perspective view of a reaction channel,

FIG. 5a a view of a honeycomb element,

FIG. 6 a perspective view of the assembled three channels according toFIGS. 3 to 5,

FIG. 7 a perspective view of extended pyrolysation, mixing and reactionchannels,

FIG. 8 a dual substance nozzle appliance with a three-way valve in theworking position,

FIG. 9 a dual substance nozzle appliance with a three-way valve in theblow-out position,

FIG. 10 an enlarged cross section through a dual substance nozzle head,

FIG. 11 a boiler installation, and

FIG. 12 a diagram for recording the signal of the urea feed quantity.

FIG. 1 shows the first compact option, and FIG. 2 shows a secondextended option for an exhaust gas cleaning installation 10 as anoverview.

In the compact option according to FIG. 1, three channels are arrangedin a housing 12, a pyrolysation channel 14, a mixing channel 16 and areaction channel 18.

In a tank installation 20 with a reagent tank, a urea granulate with agrain diameter of around 2 mm is dissolved in water in a predeterminedconcentration and directed to a metering system 22. This sends thedilute urea solution, the reagent, with slight delivery pressure into aurea conductor which is carried coaxially in a casing tube 24.Compressed air at an overpressure of around 1 bar is also supplied incasing tube 24, and flows around the urea conductor. The dilute ureasolution and the compressed air are directed into a dual substancenozzle appliance 26, from where they emerge from a nozzle which is shownbelow in detail and form an aerosol-type atomizing cone 28.

The dual substance nozzle appliance 26 protrudes into the pyrolysationchannel 14, where the finely sprayed urea solution is directed into thehot exhaust gas flow 30 which is designated with arrows. The supply line32 for the crude exhaust gases opens out into the pyrolysation channel14 in the flow direction directly in front of the dual substance nozzleappliance 26. The pyrolysis, in other words the decomposition of theurea into ammonia and carbon dioxide, if applicable with the formationof cyanuric acid, takes place immediately in the pyrolysation channel 14and continues until exhausted. The exhaust gas flow 30 with fineparticles of ammonia and carbon dioxide subsequently moves into themixing channel 16 and runs in the opposite direction through threenozzle mixers 34 of normal construction.

The exhaust gas flow 30 which has now been homogeneously mixed with thedecomposed reagent is diverted into the reaction channel, where it isfirst directed through two reduction catalysts 36 located at a distance,then through an oxidation catalyst 38 of geometrically similar designalso located at a distance, which may also be removed thereby excludingthe oxidation process. The exhaust gas flow 30 which has now beenrelieved of all gaseous effluents flows into a heat exchanger or isexpelled via a chimney 40.

For the reduction and oxidation catalysts 36, 38 shown, it is indicatedthat they are of honeycomb structure with channels running lengthwise. Aperforated metal sheet 35 as a flow rectifier is fitted before the firstreduction catalyst 36. The exhaust gas flow 30 moves in a steady flow inthe area of catalysts 36, 38, and in a turbulent fashion in the areas 42between the catalysts 36, 38. Subsequent mixing takes place in eachturbulent zone.

An electrical installation control unit 44 is usually located in a steelcabinet and it monitors and controls all the functions of theinstallation. The control unit is designed in such a way that theexhaust gas cleaning installation 10 may be operated fully automaticallyand all systems of the exhaust gas cleaning installation and the controlunit of the exhaust gas producing installation communicate with oneanother automatically. Examples include a tank agitator, a tank heater,temperature and pressure measurement sensors, a tank level probe, apump, a solenoid valve, a regulator valve, a regulator for the ureasolution, a compressed air solenoid valve and a nozzle air compressor.

The reagent regulator valve for the dilute urea solution regulates theelectrically measured actual flow value to the prescribed nominal value.The reagent regulator is a microprocessor control for the regulatorvalve or a metering pump, the engine producing the exhaust gas may bedriven at variable power. The flow of urea solution is thus controlleddependent on the power signal according to a freely-programmable NOxmass flow curve. Four different curves may be programmed and requested.The installation control unit is preferably designed according toEuronorm EN 60204.

For cleaning exhaust gases from diesel engines, a urea mass flow nominalvalue curve is programmed as a function of the engine power signal orpreferably of the electrical generator power signal. The nominal valuecurve, also designated the regulation curve, may be calculated andtraced on the basis of power points.

The nominal value is compared with the actual value in a regulator andfrom this a setting signal for the urea regulation is formed. Theregulation may be affected by a signal originating purely from NOxmeasurement on the gas, in such a way that the NOx values are kept at aconstant higher level by a slight under-stoichiometric ureaproportioning, than would be possible based on the nominal value curve.This prevents too much reagent being fed in when the SCR catalysts age,and thus NH₃ slippage occurring. It is important that the nominal valuesignal from the said regulator operates as an overriding guide signal inorder to level out fast load changes in the diesel engine sufficientlyquickly. The NOx measurement is too sluggish for this and may only beused on a subordinate basis.

The electrical installation control unit 44 is linked to the saidsections of the installation via conductors, for example via a conductor46, shown as a dashed line, with tank installation 20 and via aconductor 48 with the metering system 22. A conductor 50 is showndotted, and this replaces all the remaining conductors which lead to theelectrical installation control unit 44.

The tank installation 20 which has already been mentioned and isfamiliar in itself, comprises a plastic reagent tank. A collectingprotective trough prevents the urea solution from leaking out if thetank has a leak. The tank is equipped with a water gauge, a mixer andheater for preparing the dilute urea solution. The urea solution levelis monitored electrically. In the case of large installations, a largeurea solution storage tank is supplemented by a transfer pump system anda urea solution tank for every day use.

The metering system 22 for the urea solution is preferably arranged inan enclosed machine cabinet and comprises the following components:filter, urea solution pump, pressure maintenance valve, electrical ureasolution regulation valve, urea solution pressure measurement,electrical urea solution throughput measurement, optical reagentmeasurement, solenoid valve, jet air compressor with suction filter,compressed air filter, pressure measurement, compressed air throughputmonitoring. The installation control unit may also bememory-programmable (stored-programmable control unit).

The hydraulic and pneumatic connections to the dual substance nozzleappliance 26 and to the tank installation 20 are produced by means ofhoses, stainless steel or aluminium pipes.

All the details mentioned with respect to the tank installation 20, themetering system 22 and the electrical installation control unit 44 arefamiliar in themselves and not therefore shown in the drawings.

According to the design shown in FIG. 2, the exhaust gas flow 30 istaken as in FIG. 1 in-line through a pyrolysation channel 14, a mixingchannel 16 and a reaction channel 18. In FIG. 2 these channels arehowever not arranged in compact fashion, but extended. The urea solutionis injected as a fine atomising cone 28 towards the exhaust gas flow 30into the pyrolysation channel 14, flows via three nozzle mixers 34through the mixing channel 16 and is directed in the reaction channel 18of an essentially larger cross section initially through three reductioncatalysts 36 and then through an oxidation catalyst 38.

The tank installation 20, the metering system 22 and the electricalinstallation control unit 44 are as shown in FIG. 1.

FIG. 3 shows a pyrolysation channel 14 in perspective view, with thecover removed. The exhaust gas flow 30 is directed in from the front viaa feed pipe 32. The dual substance nozzle appliance 26 isflange-connected adjacent to this end, whereby the nozzle projects intothe inside of the pyrolisation channel 14. The dual substance nozzleappliance 26 will be described in detail later.

It is again shown that the atomizing cone 28 of the fine urea solutionis injected towards the exhaust gas flow 30. The urea solutiondecomposes immediately and completely in the hot exhaust gas flow 30 asdescribed above. The exhaust gas flow 30 is diverted on a baffle plate52 towards an outlet opening 54 as shown in FIG. 6 into the mixingchannel shown in the next FIG. 4. The gas flow 30 flows over an inletopening 56, guided by a baffle plate 58, which is flush with the baffleplate 52 according to FIG. 3, into the mixing channel 16. Two nozzlemixers are arranged in this channel. The exhaust gas flow 30 must flowthrough the nozzle mixer 34 and cannot penetrate through along the wallsof mixing channel 16.

The exhaust gas flow 30 is diverted upwards via a further baffle plate60. This baffle plate 60, together with all the other baffle plates, maybe replaced by other suitable means or omitted altogether.

The pyrolysation channel 14 which is open at the top and the mixingchannel 16 are covered with a reaction channel 18 which extends overboth channels, as shown in FIG. 5, with the exception of an inletopening 62. The exhaust gas flow 30 is diverted to the horizontal againwith a baffle plate 64 which extends over the entire width of thereaction channel 18, and initially flows through a reduction catalyst 36and then an oxidation catalyst 38. The cleaned exhaust gas flow 30 isexpelled to the open air via a tube-shaped chimney 40.

A dashed line indicates that the catalysts 36 and 38 are arranged inmodular fashion, in this embodiment they each comprise six honeycombelements. One module for example has a cross section of 150×150 mm andis 50, 150 or 300 mm long. Several honeycomb elements may be arrangedadjacently, on top of one another and/or behind one another.

The lengthwise channels are appropriately designed square, with sidelengths of 2 to 8 mm. Thus a steady flow may be produced along thelengthwise channels.

The number, arrangement and length of the modules is determined on thebasis of both technical and financial factors. Modules which are toolarge can lead to considerable manufacturing problems.

In the reduction stage, the extruded honeycomb elements are for examplemade of titanium, tungsten or vanadium oxide, or else an inert basicstructure is coated with at least one of these materials. Apart from thecatalyst composition, the channel opening and the number of channels,the web width and the number of rows of honeycomb elements may varyaccording to the design.

The reagents nitric oxide, ammonia and oxygen from the exhaust gas flow30 which flows steadily through the lengthwise channels of the honeycombstructures reach the fine-pored structure of the honeycomb walls throughdiffusion and adsorption, and react there on the active centres throughselective catalytic reduction. The reaction products are water, nitrogenand carbon dioxide.

The catalyst gradually loses its activity over a long operational periodthrough pore blockage and/or catalyst toxins which destroy the activecentres. When a minimum level of activity is reached, the catalysthoneycomb elements are replaced and recycled. The loss of activity andthe timing of replacement may be calculated.

The oxidation catalysts 38 comprise extruded honeycomb elements withceramic basic structure, into the surface of which a catalyticallyactive precious metal, for example platinum, rhodium and/or palladium,is embedded. The geometry of the honeycomb elements and the type ofactive coating may be selected for the specific application.

In contrast to the selective reduction process, the oxidation proceedswithout any supplementary reagent. The oxidation catalyst causes theoxidisable gaseous toxic exhaust gases to burn flamelessly even at verylow concentration and well below the self-combustion temperature. Forthis oxidation process it is necessary that the exhaust gas must containa minimum proportion of residual oxygen. The reaction products of theoxidation are carbon dioxide and water.

The transport of the gas molecules to be oxidised from the lengthwisehoneycomb channel into the active coating takes place, as in the case ofthe selective catalytic reduction catalysts, by diffusion, caused by thereduction in concentration in the area of the active coating.

The oxidation catalyst is also subject to ageing, it gradually loses itsactivity as a result of catalyst toxins and coatings. Once it hasreached a minimum level of activity, the catalyst honeycomb elements arereplaced and recycled.

FIG. 5a shows a front view of a honeycomb element 66, which isdesignated a module for reduction or oxidation catalysts 36, 38. Thesquare lengthwise channels 68 have an inside width of 4 to 64 mm²,dependent on their number per module cross section area, in this case 15mm².

The honeycomb element 66 is sheathed more or less flush with its lengthby an elastic hose 70 made of knitted or woven glass fibres. This hose70 serves firstly to protect it against impacts and secondly seals theadjacent honeycomb elements 66, which prevents the exhaust gas flow 30from flowing through adjacent to the honeycomb or lengthwise channels68.

According to further design variations, the pyrolysis channel 14 and themixing channel 16 for example may be designed extended, and then theexhaust gas flow 30 may be diverted into a reaction channel lying on topof the mixing channel 16 (semi-compact). Also in a compact design, thepyrolysation channel 14 may be arranged on top and the reaction channel18 underneath. Some or all of the channels may run in a direction otherthan horizontal, e.g. vertical. The inlet for the exhaust gas flow 30may be from the right or left, top or bottom, as may the outlet for thecleaned exhaust gases.

The design form according to FIG. 7 is essentially the same as that inFIG. 6, however the pyrolysation channel 14, the mixing channel 16 andthe reaction channel 18 are not designed compact, but extended. Pdesignates the pyrolysation segment, m the mixing segment and r thereaction segment.

A soot filter, not shown, made for example of aluminium oxide, glass orceramic fibres, may be connected before the pyrolysation channel 14 withthe dual substance nozzle appliance 26, as in all other design options.FIG. 8 shows a dual substance nozzle appliance 26, which is connecteddirect (FIG. 9) or via a flange 72 and a retaining bracket 74 with thewall 76 of the pyrolysation channel 14 (FIGS. 1, 2).

A two-part nozzle 78 with the atomising cone 28 as mentioned is shown indetail in FIG. 10.

A removable valve part according to FIGS. 8, 9 essentially comprises anengine housing 80 with a servomotor 82, a synchronous motor and anadjustment gear 84 and a valve housing 86 with a three-way ball valve 88with the corresponding supply lines.

The electrical conductors not shown are brought in via cable plug screwconnections 90. The servomotor 82 acts on the three-way ball valve via amotor shaft 92 and a coupling 94, fitted with contact lever 96 and limitswitch 98.

A retaining flange 100 with coaxial pipe holder 102 and support ring 104is screwed to the valve housing 36. Retaining flange 100, pipe holder102 and support ring 104 are sealed from one another with O-rings.

The casing tube 24 which leads into the area of nozzle 78 and which isconnected with the flange 72 or the wall 76 of the pyrolysation channel,is welded to the support ring 104. The casing tube 24 in this case issheathed with an insulation layer 106 which is stripped off in the areaof nozzle 78, in the area of the hot exhaust gas flow 30. The insulationlayer 106 may be omitted completely or in part.

The urea conductor 108, fixed in the pipe holder, runs inside the casingtube 24 at a distance, preferably coaxially.

The urea solution 110 is directed with low delivery pressure via a screwconnection 112 and via a connection 114 which leads to the three-wayball valve 88.

The compressed air 124 is introduced via a screw connection 116, whichleads to a cavity 118 in the support ring 104. This cavity 118 isconnected via a compressed air line 120 with a connecting piece 122which is diametrically opposite the inflow connecting piece 114 for theurea solution with respect to the three-way valve.

FIG. 8 shows the three-way ball valve 88 in its working position, it isopen in the direction of the inflowing urea solution 110. The ureasolution 110 may flow in the direction of the pipe holder 102 and may bedirected to the nozzle 78 via the urea conductor 108. The compressed air124 flows into cavity 118 and there, continuously surrounding the ureaconductor 108 as a coolant, flows in casing tube 24 to nozzle 78. No airmay flow through compressed air line 120, because the three-way ballvalve 88 is closed.

In FIG. 9, the three-way ball valve 88 is in the blow out position. Thesupply of urea solution 110 is interrupted, the compressed air may nowflow into the urea conductor via the compressed air line 120 and thethree-way ball valve 88 and may blow it out.

The nozzle 78 shown in FIG. 10 comprises a nozzle body 128 which isscrewed into a nozzle head 126 with a narrowing outlet opening 130 forthe urea solution 110. A nozzle cap 132 is screwed over nozzle body 128,and these nozzle parts form outlet channels 134 for the compressed air124. The compressed air emerging causes an aerosol-type atomization ofthe dilute urea solution through the injector effect.

An air space 136, limited by a nozzle head cover 138, is formed undernozzle 78, which continues in the direction of the outlet channels 134and is sealed with suitable means.

In the area of the outlet channels 134, a rotating movement of theemerging compressed air 124 may be caused by means of inclined grooves,inclined channels or other known means.

If a blockage of nozzle 78 and/or of the urea conductor cannot be blownout using compressed air 124, then the nozzle cap 132, possibly nozzlebody 128 and if necessary also the valve part of the retaining flange100 may be unscrewed for mechanical cleaning.

FIG. 11 shows a boiler unit 140 with an exhaust gas cleaninginstallation 10 mounted outside the boiler body 142, which essentiallycomprises a pyrolysation channel 14 with a dual substance nozzleappliance 26, a mixing channel 16 and a reaction channel 18 withoutoxidation catalyst. The exhaust gas flow 30 which is freed of toxicgases is directed into a chimney 40 through the boiler body 142 with aheat exchanger. For precipitation of fine particles suspended in theexhaust gas flow 30, the exhaust gas flow may first be directed througha fan or similar, which is not shown.

When operating a boiler or other combustion units with sulphurous fuels,e.g. heavy fuel oil, minimum operating temperatures which depend on thesulphur content must be observed. If temperatures are too low, ammoniumsulphate is formed, which firstly coats the catalyst with an adhesive,viscous layer and secondly is highly corrosive. The following minimumtemperatures have been determined, which are reached by a suitableburner unit 146, by means of heat exchangers and/or positioning of theexhaust gas cleaning installation 10 outside or inside the boiler body142:

0.05% in weight sulphur=513° K.

0.05% in weight sulphur=533° K.

0.3% in weight sulphur=553° K.

0.5% in weight sulphur=563° K.

1% in weight sulphur=573° K.

1.5% in weight sulphur=613° K.

2% in weight sulphur=623° K.

3% in weight sulphur=693° K.

4% in weight sulphur=713° K.

The minimum temperatures may also be increased by mixing hotter exhaustgases from the boiler, before bringing the catalyst into use. Themixed-in exhaust gas flow should always be smaller or at least of thesame size as the main exhaust gas flow. The exhaust gas flow may becontinuously regulated by a motor-driven valve. The temperature may berecorded by means of a thermostat, which gives a signal to theregulator.

FIG. 12 shows a diagram of the signal recording for the urea meteringquantity. Fuel 147 is directed to a combustion installation 148, asshown with an arrow. The uncleaned exhaust gases are directed via a line32 to an exhaust gas cleaning installation 10 with a reduction catalyst36 or a reduction catalyst and an oxidation catalyst 38, and the exhaustgas flow 30 cleaned of toxic gases emanates from this installation. Froma tank installation 20 shown in more detail in FIG. 1, the reagent isdirected into the exhaust gas cleaning installation 10 via a regulatorof the electrical installation control unit 44, with a metering system22 and a casing tube 24.

The regulation signals or their conductors from the electricalinstallation control unit 44 are not shown, they may be taken from FIGS.1 and 2.

In the case of solid fuels 147, a line 149 branches to a gas analysisinstrument 150 for chemical/physical analysis of the pipe with thecleaned exhaust gas flow 30 leading to a heat exchanger or a chimney.This produces signals in a known manner, which are directed to theelectrical installation control unit 44 via an electrical conductor 151.

In the case of liquid or gas fuels 147, this is recorded using aphysical measuring instrument 152, which also produces signals andpasses them to the electrical installation control unit 44, theregulator, via a further electrical conductor 153.

As the reduction of nitrous oxides in the exhaust gas 30 of combustioninstallations 148 with excess oxygen only takes place by theproportioned supply of dilute urea solution 110 (FIGS. 8, 9), thenominal value of the supply quantity must be recorded. The level ofefficiency of the exhaust gas cleaning installation 10 depends on theprecise recording of the input signal of the toxic gases to beeliminated from the exhaust gas flow 30. If the recording is undertakenby a gas analysis measuring instrument 150, which measures the toxicgas, then the level of efficiency of the exhaust gas cleaninginstallation 10 is mainly determined by the measurement accuracy of thismeasuring instrument and its constancy over time. In the case of gasanalysis, several physical parameters of the individual gas concernedand their cross-sensitivity to further individual gases and the physicalvalues must be determined for precise chemical and physical definitionof the actual value.

In the case of oil or gas-fired combustion installations 148, the fuelconstituents are largely constant and thus directly proportional totheir exhaust gas components. The nominal value for the addition of thereduction agent, the dilute urea solution, may be based on the signalstransmitted by the physical measuring instrument 152, and the gasanalysis measuring instrument 150 is not needed.

If recording takes place via the physical measuring instrument 152 bymeans of a meter or a potentiometer, the signal recorded may be used asan actual value for the addition of the dilute urea solution. The actualvalue signal may be transferred direct or with an electrical converteras a 0-20 mA or 4-20 mA signal to the electrical installation controlunit 44 or to an actuator which is not shown. Without recalibration, theerror limit after six months should not exceed ±2%.

The dependence of the development of toxic gases on non-fuel-relatedactual values is assessed by periodic gas analysis measurement. Anactuator may be triggered by the input into the electrical installationcontrol unit 44 of a group of curves of the regulation parameters thusrecorded.

The fuel data and non-fuel-related data are recorded as described andfed into the regulation unit 44, which acts on actuators. The electricalsignals may directly trigger a permanently regulated pump, and be inputas a nominal value, instead of triggering the regulation unit 44. A gasanalysis signal may additionally be adapted to an actual value signal.

Solid fuels have varying fuel constituents. They are burned incombustion installations 148 with large furnace bodies. The yield of thesolid matter and the consumption can scarcely be properly determined, orat least cannot be determined in a homogeneous manner. The quantity oftoxic constituents in the exhaust gas flow is also variable, inaccordance with the variable fuel composition. The fuel quantity istherefore not suitable as an actual value signal for the reagent flow.As a constant signal, the volume flow of the combustion air is recordedas a physical value. The physical measuring instrument 152 and acontinuous measurement gas analysis instrument 150 record the nitricoxide content in ppm. Both signals are recorded via a computer asnitrous oxide mass flow and are used as an actual value for the meteringof dilute urea solution, via a 0 to 20 mA or 4 to 20 mA analog signal.

We claim:
 1. A process for removing effluents from an exhaust gas streamcomprising the steps of:providing a pyrolysation channel fortransporting the exhaust gas stream along a first direction; providing adilute urea solution mixed with compressed air for controlling thetemperature of the dilute urea solution to a maximum temperature ofabout 100° C. so as to prohibit decomposition of the urea prior toinjection; injecting into said exhaust gas stream through the nozzle insaid first direction an aerosol mixture comprising a dilute ureasolution mixed with compressed air wherein said urea decomposes intoammonia and carbon dioxide; providing mixing means in said pyrolysationchannel downstream of said nozzle with respect to said first directionfor forming a homogeneous mixture of the exhaust gas with said ammoniaand carbon dioxide; providing a catalyst means in said pyrolysationchannel downstream of said mixing means with respect to said firstdirection; and passing said homogeneous mixture through said catalystmeans for removing effluents therefrom.
 2. A process according to claim1 wherein said catalyst means comprises a non-zeolite reductioncatalyst.
 3. A process according to claim 1 wherein said catalyst meanscomprises an oxidation catalyst.
 4. A process according to claim 3wherein the oxidation catalyst is SOx selective.
 5. A process accordingto claim 1 wherein said catalyst means comprises a combination of anon-zeolite reduction catalyst and an oxidation catalyst arranged inseries.
 6. A process according to claim 5 including the step ofproviding means between said serially arranged catalysts for providingturbulent flow of said homogeneous mixture.
 7. A process according toclaim 1 wherein the aerosol mixture is injected into an exhaust gasstream flowing at a flow rate of between 15-60 meters per second at adelivery pressure of between 0.2 to 8 bars.
 8. A process according toclaim 1 wherein the dilute urea solution is rotated by the compressedair after leaving the nozzle.
 9. A process according to claim 1including the steps of providing the dilute urea solution instoichiometric amounts with respect to the NOx present in the exhaustgas stream.
 10. A process according to claim 1 further including thesteps of injecting solely compressed air into said pyrolysation channelthrough said nozzle prior to feeding the dilute urea solution to saidnozzle.
 11. A process according to claim 10 further including the stepsof automatically cutting off the flow of the dilute urea solution to thenozzle when the feed of compressed air to the nozzle falls below apresubscribed minimum.